That's true with a perfectly clean new chain and the way the Gates system is recommended to be set up (with a tension preload). If the belt drive system were to use no pre-load and a tensioner instead, the belt system would become more efficient once the rider starts putting out 200 watts or more.

Again, that's with a new, clean chain. To me a belt drive is attractive because it is less effected by exposure to the elements. My chains can be in pretty rough shape during the winter between cleanings and I'm guessing a rubber band might be more efficient at times.